Project Summary/Abstract Prosthetic alignment is an important aspect in the successful rehabilitation of lower-limb amputees and is a deciding factor in whether the prosthetic limb will restore function, minimize gait deviations, and be comfortable for the user. Prosthetists are trained to follow an iterative procedure when aligning a prosthetic limb that relies on clinical experience and patient feedback. The prosthetist continually modifies prosthetic alignment until they are satisfied that the patient exhibits few gait deviations and is comfortable. However, this process suffers from poor repeatability and little scientific justification. In order to improve the process of transfemoral alignment and promote an objective basis for practice, researchers and clinicians must first acquire a better understanding of the relationship between variations in prosthetic alignment and the biomechanical response of the amputee. Sagittal instability of the limb is a primary concern for transfemoral amputees because they do not have direct control of their prosthetic knee joint. As a result, transfemoral amputees rely on muscle strategies at the hip to control knee flexion and extension. A practical consequence of prosthetic misalignment is that it may require the amputee to generate excessive hip moments to control their knee joint, resulting in increased metabolic energy expenditure and fatigue during walking. Improper alignment may also contribute to gait asymmetries, poor performance of prosthetic components, and undesirable pressure between the socket and residual limb, giving rise to discomfort and pressure sores. The purpose of this study is to evaluate how gait biomechanics of persons with unilateral, transfemoral amputations are altered in response to systematic changes in sagittal- plane alignment. Both linear translations of the prosthetic knee and rotations of the prosthetic foot will be investigated during straight, level walking. To measure the response of transfemoral amputees to variations in sagittal-plane alignment, we will collect kinematic and kinetic data using quantitative gait analysis. We will also acquire EMG data of hip musculature, intrasocket contact pressure, and energy expenditure measurements. This proposed work will characterize the relationship between prosthetic misalignment and the functional response of transfemoral amputee gait, including biomechanical asymmetries, increased energy expenditure, and residual limb pain. The results of this study will provide a thorough understanding of how transfemoral amputees control their prosthetic knee joint and may establish a more systematic approach to the process of clinical prosthetic alignment.